Little is known about the nature of mutation at the DNA sequence level in mammalian cells. The emerging new biology utilizing recombinant DNA technology makes it possible to investigate mutagenesis at the DNA level in the mammalian genome. This project is directed at that goal and involves. (1) The construction of appropriate shuttle vectors; (2) The development of genetic mapping systems for the pre-localization of mutations; and (3) The development of recovery and sequencing technology for the characterization of mutants.